MX2012008177A

MX2012008177A - Process of supplying water of controlled salinity.

Info

Publication number: MX2012008177A
Application number: MX2012008177A
Authority: MX
Inventors: John Dale Williams
Original assignee: Bp Exploration Operating
Priority date: 2010-01-14
Filing date: 2011-01-11
Publication date: 2012-10-01
Also published as: AU2011206433A1; EA201201005A1; AU2011206433B2; CN102803148B; BR112012017561B1; US20150083656A1; MX347833B; CN102803148A; US9492790B2; BR112012017561A2; CA2786348A1; CA2786348C; EP2523908A1; EA025116B1; US20120261340A1; US9555373B2; WO2011086346A1; EP2523908B1

Abstract

A process of producing an injection water stream of controlled salinity and controlled sulfate anion content that is suitable for injection into an oil bearing formation of an oil reservoir, the process comprising the steps of: (a) feeding a source water having a total dissolved solids content in the range of 20,000 to 45,000 ppm and a sulfate anion concentration in the range of 1,000 to 4,000 ppm, preferably, 1,500 ppm to 4,000 ppm to a desalination plant that comprises a plurality of reverse osmosis (RO) membrane units and a plurality of nanofiltration (NF) membrane units wherein the source water is pressurised to a pressure in the range of 350 to 1250 psi absolute, and dividing the source water to provide a feed water for the RO membrane units (hereinafter "RO feed water") and a feed water for the NF membrane units (hereinafter "NF feed water"); (b) if necessary, increasing the pressure of the RO feed water to a value in the range of 900 to 1250 psi absolute before introducing the RO feed water to the RO membrane units and withdrawing an RO permeate and an RO retentate from the RO membrane units wherein the RO membrane units are operated in either a single-pass, single-stage mode or in a single-pass, two-stage mode and wherein the recovery of RO permeate is in the range of (35) to 75% by volume, preferably, 35 to 60% by volume based on the volume of the RO feed water that is fed to the RO membrane units such that the RO permeate has a total dissolved solids contents of less than 250 ppm, and a sulfate anion concentration of less than 3 ppm; (c) if necessary, reducing the pressure of the NF feed water to a value in the range of 350 to 450 psi absolute before introducing the NF feed water to the NF membrane units and withdrawing an NF permeate and an NF retentate from the NF membrane units wherein the NF membrane units are operated in a single-pass, single-stage mode and wherein the NF membrane units are operated with a recovery of NF permeate in the range of 35 to 60% by volume based on the volume of the NF feed water that is fed to the NF membrane units such that the NF permeate has a total dissolved solids content in the range of 15,000 to 40,000 ppm, preferably, 15,000 to 35,000 ppm, and a sulfate anion concentration of less than 40 ppm, preferably less than 30 ppm; and (d) mixing at least a portion of the RO permeate and at least a portion of the NF permeate in a ratio in the range of 2: 1 to 40: 1, preferably, 4: 1 to 27:1, in particular, 10: 1 to 25: 1 to provide an injection water having a total dissolved solids content in the range of 500 to 5,000 ppm, and a sulfate anion concentration of less than 7.5 ppm, preferably, less than (5) ppm, more preferably less than 3 ppm.

Description

PROCEDURE FOR SUPPLYING CONTROLLED SALINITY WATER FIELD OF THE INVENTION The present invention relates to a method for providing a low salinity injection water for an oilfield having sufficient salinity to avoid formation damage and a sulphate anion concentration low enough to prevent acidification of the deposit, and a desalination system to produce an injection water of this type. In particular, the present invention provides a method and system for producing water of low controlled salinity, concentration of controlled sulfate anions and concentration of controlled multivalent cations.

BACKGROUND OF THE INVENTION As described in the international patent application WO 2008/029124, it is known to inject low salinity water into a reservoir oil formation in order to improve oil recovery from the reservoir.

A problem associated with low salinity water injection is that desalination techniques can produce water that has a salinity lower than optimum salinity for improved oil recovery. In fact, desalinated water can damage the reservoir's oil rock formation and inhibit oil recovery, for example, by producing clay expansion in the formation. There is an optimum salinity for injection water that provides the benefit of improved oil recovery while avoiding damage to the formation, and, the optimum value will vary from one formation to another. Normally, when an oil formation comprises rock containing high levels of expansive clays, damage to formation can be avoided when the water injected has a total dissolved solids (TDS) content in the range of 500 to 5,000 ppm, preferably from 1,000 to 5,000 ppm.

However, it is not desirable to mix a desalinated water of low content in multivalent cations with a high salinity water such as seawater due to the high content of sulfate anions and / or high multivalent cation content of the high salinity water. Therefore, the high sulfate anion content of mixed water streams of this type can result in acidification of the deposit and / or precipitation of unacceptable levels of insoluble mineral salts (scale formation) when the injected water comes in contact with precipitated precursor cations, such as barium, strontium and calcium cations that are commonly present in the fossil water of the formation. In addition, mixing the desalinated water with a high salinity water such as seawater can result in the mixed water stream containing unacceptable levels of multivalent cations, in particular calcium and magnesium cations. Therefore, in order to achieve gradual oil recovery with an injection water of low salinity, the ratio of the concentration of multivalent cations in the water of low salinity injection with respect to the concentration of multivalent cations in the fossil water of the reservoir should be less than 1, preferably less than 0.9, more preferably less than 0.8, in particular less than 0.6, eg, less than 0.5.

As described in the international patent application WO 2007/138327, one way in which the salinity of a water supply of too low salinity could be increased is combined with water of higher salinity. According to WO 2007/138327, this can be achieved by the steps of: substantially desalting a first water feed supply to provide a first supply of treated water of low salinity; treating a second water feed supply to provide a second supply of treated water having a reduced concentration of divalent ions compared to the second feed supply and a higher salinity than the first water supply treated; Y mixing the first treated water supply and the second treated water supply to provide a mixed water supply having a desired salinity suitable for injection into an oilfield.

In preferred embodiments of the invention of WO 2007/138327, the first feed supply desalts substantially by a reverse osmosis process, while the step of treating the second feed water supply is preferably performed by nanofiltration.

Nanofiltration is commonly used in the petroleum industry to remove sulfate ions from a source water. The treated water can then be injected into a formation without the risk of forming unacceptable levels of insoluble mineral salts when the injected water comes into contact with precursor precipitate cations present in the fossil water of the formation. The invention of WO 2007/138327 therefore allows the supply of a mixed water having the desired salinity suitable for injection into the oil field and having a reduced level of sulfate anions thereby mitigating the risk of mineral scale precipitation. either within the training or in the production wells.

It is known that injection of a water containing high levels of sulfate anions can stimulate the growth of sulfate-reducing bacteria that produce hydrogen sulfide as a metabolite resulting in the acidification of a reservoir. When it is desired to mitigate the risk of mineral scale formation, the level of sulfate anions in the mixed water supply should be less than 40 ppm. However, when it is desired to mitigate the risk of acidification in a reservoir, the level of sulfate anions in the mixed water supply should be as low as possible, for example, less than 7.5 ppm, preferably less than 5 ppm.

It has now been found that it is necessary to carefully control the operating conditions of the process of WO 2007/138327 in order to achieve a mixed water supply of total dissolved solids content desired to control formation damage and low concentration of water. sulfate anions desired to control the acidification of the deposit.

SUMMARY OF THE INVENTION The present invention therefore relates to an improved process and plant for providing a mixed water stream of controlled salinity, and a low content of sulfate anions controlled for use as injection water for a low salinity water injection while it mitigates the risk of damage to the formation, and to control the acidification in the deposit.

Therefore, according to a first embodiment of the present invention there is provided a process for producing a water stream of controlled salinity injection and controlled sulfate anion concentration which is suitable for injection into an oil formation of an oil field, the process comprising the steps of: feeding a source water having a total dissolved solids content in the range of 20,000 to 45,000 ppm and a concentration of sulfate anions in the range of 1,000 to 4,000 ppm, preferably from 1,500 ppm to 4,000 ppm to a plant of desalination comprising a plurality of reverse osmosis membrane units (01) and a plurality of nanofiltration membrane (NF) units, wherein the source water is pressurized to a pressure in the range of 350 to 1250 psi absolute , and divide the source water to provide a feedwater for the RO membrane units (in hereinafter "OI feed water") and a feed water for the NF membrane units (hereinafter "NF feed water"); if necessary, increase the pressure of the RO feed water to a value in the range of 900 to 1250 psi absolute before introducing the RO feed water into the RO membrane units and removing a permeated RO material and a Retained OI material from OI membrane units, wherein the OI membrane units are operated either in a single-pass, single-phase or single-pass, two-phase mode and in which the recovery of permeated RO is in the range from 35 to 75% by volume, preferably from 35 to 60% by volume based on the volume of the OI feed water that is fed to the RO membrane units, so that the RO permeated material has a solids content dissolved totals less than 250 ppm, and a sulfate anion concentration less than 3 ppm; if necessary, reduce the NF feed water pressure to a value in the 350 to 450 psi absolute range before introducing the NF feed water into the NF membrane units and removing an NF permeate material and a NF retained material of the NF membrane units, wherein the NF membrane units are operated in a single-pass, single-phase mode and wherein the NF membrane units are operated with a recovery of NF permeate material in the range of 35 to 60% by volume based on the volume of the NF feedwater that is fed to the NF membrane units, so that the permeate material of NF has a total dissolved solids content in the range of 15,000 to 40,000 ppm, preferably 15,000 to 35,000 ppm, and a sulfate anion concentration of less than 40 ppm, preferably less than 30 ppm; Y mixing at least a portion of the RO permeate material and at least a portion of the NF permeate material in a ratio in the range of 2: 1 to 40: 1, preferably 4: 1 to 27: 1, in particular 10: 1 to 25: 1 to provide an injection water having a total dissolved solids content in the range of 500 to 5,000 ppm, preferably 1,000 to 5,000 ppm, and a sulfate anion concentration of less than 7.5 ppm, preferably lower at 5 ppm, more preferably less than 3 ppm.

The source water can be seawater, estuarine water, produced water, aquifer water, or wastewater.

Preferably, the total dissolved solids (TDS) content of the permeate material of 01 is in the range of 50 to 225 ppm, more preferably 100 to 225 ppm, most preferably 125 to 200 ppm, in particular 150 to 175 ppm. .

Preferably, the concentration of sulfate anions of the permeated OI material is in the range of 0.5 to 2.5 ppm, in particular 0.5 to 1.5 ppm.

Preferably, the TDS of the NF permeate material is not more than 15,000 ppm lower, preferably not more than 10,000 ppm lower than the TDS of the source water.

Preferably, the concentration of sulfate anions of the permeate material of NF is in the range of 10 to 28 ppm, more preferably 10 to 25 ppm, in particular 15 to 20 ppm.

The concentration of sulfate anions in the injection water will depend on the desired content of total dissolved solids (TDS) for this stream and therefore on the mixing ratio for the permeate material of OI and the permeate material of NF. Therefore, the sulfate anion concentration of the injection water will increase with increasing amounts of NF permeate in the mixed stream. Normally, the concentration of sulfate anions for an injection water stream having a total dissolved solids content of 1000 ppm is in the range of 1 to 2 ppm, and the values for the range for the concentration of sulfate anions should be adjusted to scale for higher TDS injection waters.

An advantage of the process of the present invention is that in addition to providing an injection water having a TDS high enough to mitigate the risk of formation damage and having a sulfate concentration sufficiently low to mitigate the risk of acidification in the reservoir, depending on the choice of source water, the injection water may also have a concentration of multivalent cations sufficiently low for use as low salinity injection water thereby achieving gradual oil recovery from the reservoir.

Accordingly, the present invention also relates to an improved process and plant for providing a mixed water stream of controlled salinity, low concentration of controlled sulfate anions and controlled multivalent cation concentration for use as injection water for an injection of water of low salinity while mitigating the risk of damage to the formation, and to control the acidification in the deposit.

Therefore, in a second embodiment of the present invention, there is provided a method of producing a controlled salinity injection water stream, controlled sulfate anion concentration and controlled multivalent cation concentration which is suitable for injection into an oil formation ( of an oil field, the method comprising the steps of: feeding a source water having a total dissolved solids content in the range of 20,000 to 45,000 ppm, a sulfate concentration in the range of 1,000 to 4,000 ppm, preferably 1,500 ppm at 4,000 ppm, and a concentration of multivalent cations in the range of 700 to 3,000 ppm, preferably from 1,000 to 3,000 ppm, more preferably from 1,500 to 2,500 ppm to a desalination plant comprising a plurality of reverse osmosis membrane units (OI) and a plurality of nanofiltration membrane (NF) units, wherein the source water is pressurized to a value in the range of 350 to 1250 psi absolute, and split the source water to provide an OI feed water and an NF feedwater; if necessary, increase the pressure of the RO feed water to a value in the range of 900 to 1250 psi absolute before introducing the RO feed water into the RO membrane units and removing a permeated RO material and a OI retained material of the membrane units of 01, in which the OI membrane units are operated either in a single-pass mode, single phase or in a single pass mode, two phases and in which the permeate material recovery of 01 is in the range of 35 to 75% by volume, preferably 35 to 65% by volume based on the volume of the feed water of 01 that is fed to the RO membrane units, so that the permeate material of 01 has a total dissolved solids content of less than 250 ppm, a sulfate anion concentration of less than 3 ppm, and a multivalent cation content of up to 10 ppm; if necessary, reduce the NF feed water pressure to a value in the 350 to 450 psi absolute range before introducing the NF feed water into the NF membrane units and removing an NF permeate material and a NF retained material of the NF membrane units, wherein the NF membrane units are operated in a single-pass, single-phase mode with a recovery of NF permeate material in the range of 35 to 60% in volume based on the volume of the NF feedwater that is fed to the NF membrane units, so that the permeate material of NF has a total dissolved solids content in the range of 15,000 to 40,000 ppm, preferably 15,000 to 35,000 ppm, a sulfate anion concentration lower than 40 ppm, preferably lower than 30 ppm and a multivalent cation content of up to 20 (0 ppm, preferably up to 150 ppm, more preferably up to 100 ppm; mixing at least a portion of the RO permeate material and at least a portion of the NF permeate material in a ratio in the range of 2: 1 to 40: 1, preferably 4: 1 to 27: 1, in particular 10: 1 to 25: 1 to provide an injection water having a total dissolved solids content in the range of 500 to 5,000 ppm, preferably 1,000 to 5,000 ppm, a sulfate anion concentration of less than 7.5 ppm, preferably less than 5 ppm, more preferably less than 3 ppm and a multivalent cation content of up to 50 ppm.

Again, the source water can be seawater, estuary water, a produced water, an aquifer water, or a wastewater.

The preferred TDS for the source water, the RO permeate material, the NF permeate material and the injection water are as previously provided for the first embodiment of the present invention.

The source water preferably has a concentration of calcium cations in the range of 200 to 600 ppm. Preferably, the source water has a concentration of magnesium cations in the range of 500 to 2000 ppm.

The preferred concentrations (of sulfate anions in the RO permeate material, permeate NF material and injection water are as previously provided for the first embodiment of the present invention.

Preferably, the concentration of multivalent cations in the permeated RO material is in the range of 1 to 10 ppm, preferably 1 to 5 ppm, in particular 1 to 3 ppm.

Preferably, the concentration of multivalent cations in the NF permeate material is in the range of 50 to 200 ppm, preferably 50 to 150 ppm.

The concentration of multivalent cations in the injection water will depend on the TDS desired for this stream and therefore on the mixing ratio for the permeated RO material and the NF permeated material. Therefore, the concentration of multivalent cations in the injection water will increase with increasing amounts of NF permeate material in the mixed stream. Typically, the concentration of multivalent cations for an injection water stream having a total dissolved solids content of 1000 ppm is in the range of 2 to 10 ppm, and the values for the multivalent cation concentration range must be scaled. for higher TDS injection waters.

As discussed above, when it is desired to achieve gradual oil recovery with a low salinity injection water, the ratio of the multivalent cation concentration of the low salinity injection water to the concentration of multivalent cations of the fossil water must be less than 1. The concentration of multivalent cations of a fossil water is usually several times greater than the concentration of multivalent cations of the injection water formed by mixing the permeate material of OI and the permeate material of NF according to the process of the present invention . Accordingly, the injection water has the desired low salinity and the low concentration of multivalent cations desired to achieve gradual oil recovery when injected into a hydrocarbon-bearing formation of a reservoir while having a sufficient content of total dissolved solids to avoid damage to the formation and a sulphate concentration sufficiently low to mitigate the risk of acidification in the deposit (as well as to mitigate the risk of precipitation of insoluble mineral salts in the formation and / or production wells).

Normally, the formation in which the controlled salinity injection water is injected (controlled TDS), low concentration of controlled sulfate anions and controlled low concentration of multivalent cations is a sandstone oil formation containing a high content of expansive clays, for example, smectite-type clays. By high content of expansive clays is meant a content of expansive clays of 10% by weight or more, for example, a content of expansive clays in the range of 10 to 30% by weight.

Typically, in these first and second embodiments of the present invention, the RO permeated material and the permeated NF material are mixed in one volume ratio (volume of RO permeate material to volume of NF permeate) of 2. : 1 to 40: 1, in particular from 4: 1 to 27: 1, in particular from 10: 1 to 25: 1. The person skilled in the art will understand that the particular mixing ratio will depend on one or more of the following factors: (a) the salinity of the source water; (b) the sulfate concentration of the source water; (c) the concentration of multivalent cations of the source water; (d) the temperature at which the membrane units of 01 and NF are operated; (e) the percentage volume recovery to which the membrane units of 01 and NF are operated; (f) the desired salinity of the injection water; (g) the desired sulfate anion concentration of the injection water; Y (h) the desired multivalent cation concentration of the injection water.

The factors (f), (g) and (h) depend, in turn, on the characteristics of the reservoir in which it is desired to inject the treated water such as the amount of expansive clays, the levels of sulfate-reducing bacteria, and the concentration of multivalent cations of fossil water. Therefore, depending on the mixing ratio of the permeate material of 01 with respect to the permeate material of NF, the injection water stream will have a sufficient salinity to control formation damage, a sulfate concentration low enough to control the acidification in the oilfield, and a sufficiently low concentration of multivalent cations so that the ratio of the concentration of multivalent cations of the injection water with respect to that of the fossil water of the formation is less than 1.

Advantageously, the mixing ratio of the permeate material of 01 and the permeate material of NF is controlled according to a measured variable. The control can be automatic and a feedback control system can be used.

The measured variable can be a property of the injection water, for example, the measured variable can refer to the salinity (TDS content) of the injection water, and preferably it is the conductivity of the injection water. Conductivity is a measure of the TDS content of the injection water. Alternatively, or additionally, the measured variable may refer to the concentration of multivalent anions in the injection water or in the permeate material of NF, or the concentration of selected divalent anions, such as sulfate anions, in the injection water or in the permeate material of NF. Alternatively, or additionally, the measured variable may refer to the concentration of multivalent cations in the injection water or in the permeated material of NF, or the concentration of selected multivalent cations, such as calcium cations and / or magnesium cations in the water of injection or in the permeated material of NF.

The flow velocity of the injection water stream or the source water stream can also be controlled according to a measured variable.

By "single-pass, single-phase" mode it is meant that the feed water is passed through a plurality of individual membrane units which are arranged in parallel. Therefore, a feedwater is passed to each of the membrane units and a stream of permeate material and one (stream of retained material from each of the membrane units) are drawn in. The permeate material streams are combined. then to form a stream of combined permeate material The percentage recovery of the membrane units when operated in "single pass, single phase" mode is: [(volume of permeate material stream combined / volume of feeding) x 100] These volumes are determined over a set period of time, for example, volume of feedwater processed in one day and volume of combined permeate flow produced in one day.

By "single-pass, two-phase" mode it is meant that the feed water is fed to the first of two membrane units which are arranged in series, the retained material of the first membrane unit being used as feed water for the second membrane unit in the series. Normally, there may be a plurality of first membrane units that are arranged in parallel and a plurality of second membrane units arranged in parallel. Generally, there will be fewer second membrane units than first membrane units, since the second membrane units will process a smaller volume of water over a set period of time than the first membrane units. Normally, (the permeate material streams from the first membrane units are mixed to give a first stream of permeate material and the currents of retained material from the first membrane units are mixed to form a first stream of retained material. The first stream of retained material is then used as feedwater for the plurality of second membrane units which are arranged in parallel.The streams of permeate material from the second membrane units are then normally mixed to give a second stream of permeate material. The second stream of permeate material is then combined with the first stream of permeate material to give a stream of combined permeate material The streams of material retained from the second membrane units are normally mixed to give a stream of combined retained material that It is downloaded from the p desalination lanta. However, there are other ways of combining the various streams when operating a plurality of membrane units in a "single pass, two phase" mode which are within the general knowledge common to one skilled in the art.

The percentage recovery of the membrane units when operated in "single-pass, two-phase" mode is: [(volume of the first permeate material stream from the first membrane units + volume of the second material stream permeate from the second membrane units) / the volume of the feed water to the first membrane units)) x 100]. These volumes are determined over a set period of time, for example, one day.

The NF membrane units are preferably operated in "single pass, single phase" mode. The OI membrane units are preferably operated in either "single pass, single phase" or "single pass, two phase" mode, in particular, in "single pass, single phase" mode.

In the present invention, the OI membrane units are operated with a pressure differential across the membrane that provides a recovery of permeated RO material in the range of 35 to 75% by volume, preferably 35 to 65% in volume, more preferably from 35 to 60% by volume, most preferably from 45 to 55% by volume, in particular from 50 to 55% by volume, based on the volume of the OI feed water.

Normally, the pressure differential across the OI membrane units (OI feed water pressure - combined OI retained material pressure) is in the range of 25 to 100 psi, preferably 35 to 75 psi, per example, approximately 50 psi. Accordingly, the streams of retained material leaving the RO membrane units are at a relatively high pressure. Preferably, some or part of the streams of material retained by RO to be discharged from the RO membrane units can be combined and the stream of retentate material from the combined combined OI passed through a hydraulic recovery unit, for example. , a hydraulic recovery turbine or a turbocharger that is coupled to a booster pump for the RO water supply. Therefore, the hydraulic recovery unit recovers energy from the unit of material retained from OI and uses this recovered energy to reinforce the pressure of the OI feed water thereby reducing the power requirements for the desalination plant. Normally, the pressure of the combined OI material flow stream downstream of the hydraulic recovery unit is less than 100 psig, preferably in the range of 10 to 75 psig, in particular 20 to 55 psig, e.g. 50 psig.

In the present invention, the NF membrane units are operated with a pressure differential across the membrane which provides a recovery of NF permeate material in the range of 35 to 60% by volume, preferably 45 to 55% in volume, in particular about 50% by volume, based on the volume of the NF feed water.

Normally, the pressure differential across the NF membrane units (NF feed water pressure - NF retained material pressure) is in the range of 25 to 100 psi. Accordingly, the pressure of the combined NF material flow is usually too low to ensure that energy is recovered from this current. However, if desired, energy can also be recovered from the retained NF stream using a hydraulic recovery unit.

Preferably, the desalting plant comprises at least two membrane trains, preferably from 2 to 12, more preferably from 2 to 8, for example from 2 to 6, in particular from 4 to 6 membrane trains, each train comprising a plurality of OI membrane units and a plurality of NF membrane units. Normally, the ratio of OI membrane units to NF membrane units in each train is in the range of 2: 1 to 40: 1., preferably from 4: 1 to 27: 1, in particular from 10: 1 to 25: 1. Accordingly, an advantage of the desalination plant of the present invention is that a separate NF train is eliminated, which reduces space and weight considerations, which is of particular concern for the offshore installations in which the plant is located. located on a platform or a floating production storage and discharge facility (FPSO). In addition, the incorporation of NF units in each train of the desalination plant of the present invention means that injection water of the desired composition remains available even if one or more of the trains in the desalination plant is unused for cleaning, maintenance, or in the case of an emergency.

Each train can be equipped with specialized pumping systems and, optionally, specialized hydraulic recovery systems. Alternatively, there may be a common pumping system and, optionally, a common hydraulic recovery system, for the plurality of trains.

Preferably, the membrane units of each train are arranged in a plurality of rows or shelves. In order to reduce the occupied space of the desalination plant, it is preferred that these rows be arranged one above the other. Preferably, each membrane train comprises between 3 and 15 rows, preferably between 6 and 12 rows. Generally, there are between 4 and 16 membrane units, preferably between 6 and 12 membrane units in each row. Normally, the NF membrane units are arranged together, for example, all or a part of the membrane units of one or more of the rows can be NF membrane units. When the OI membrane units are operated in "single pass, two phase" mode, it is preferred that the first membrane units in the series are arranged together in one or more rows and that the second membrane units in the series They are also arranged together in one or more rows.

Preferably, the membranes of the membrane units of NF and OI are spirally wound membranes. Spirally wound membranes typically have a length in the range of 40 to 60 inches (from 1.08 to 1.52 meters) and an outer diameter in the range of 2.5 to 18 inches (from 6.36 to 45, 7 cm).

The NF membrane units and the RO membrane units of each train comprise a plurality of pressure containment housings containing at least one membrane, preferably 4 to 8 membranes. The housings may be formed of resin reinforced with glass or steel. Typically, each housing can withstand a pressure greater than 1100 psi absolute, preferably greater than 1300 psi absolute, in particular greater than 1400 psi absolute. Normally, the housings have (cylindrical shape and are arranged parallel to each other, in rows (or shelves), the longitudinal axes being located through the housings in a substantially horizontal plane.

In a first preferred aspect of the present invention, the source water can be pressurized to the desired feed pressure for the RO membrane units of each train, for example, using a high pressure pump. The source water is then divided to provide RO feed water for the RO membrane units and NF feed water for the NF membrane units. When the OI membrane units of the train are operated in single-pass, two-phase mode, the desired supply pressure for the OI membrane units refers to the pressure at which the first membrane units are operated. the Serie.

Typically, for the first preferred aspect of the present invention, each membrane train is provided with a feed manifold for the OI feed water, a feed manifold for the feed water of NF, a collector of material retained for a feed. combined retentate stream and a permeate material collector for a combined permeate stream. The supply manifold of 01 and the NF supply manifold are in fluid communication with a supply conduit for the source water. When a row only contains membrane units of 01 or only NF membrane units, a common supply conduit is provided which leads from the appropriate supply manifold (01 supply manifold and NF supply manifold, respectively) to the individual membrane units of each row. Similarly, a common retentate flux conduit and a common permeate flux conduit lead from the individual membrane units in each row to the retentate and permeate material collectors., respectively. When a row contains both membrane units of 01 and NF, a dedicated common supply conduit is provided for the membrane units of 01 leading from the 01 supply manifold and an additional specialized common supply conduit for the membrane units of NF leading from the NF feed manifold. Similarly, the OI and NF membrane units of the row may be provided with specialized common retained material conduits and specialized common permeate material flow conduits.

A flow controller can be provided in the or in each common NF feed conduit to control the separation of source water between the RO membrane units and the NF membrane units. As discussed above, the feed or inlet pressure for the NF units is in the range of 350 to 450 psi absolute, in particular 380 to 420 psi absolute, for example, of about 400 psi absolute. When the source water pressure is above the desired inlet pressure for the NF membrane units, a pressure lowering valve can be provided in the or each common NF feed line so that the pressure can be reduced up to the desired inlet pressure. Alternatively, a control valve may be provided on or in each supply conduit for the NF membrane units, the control valve regulating the source water flow to the NF membrane units and also decreasing the water pressure of the NF membrane units. origin up to the desired inlet pressure for the NF membrane units. It can also be conceived that a flow controller can be provided upstream of the NF supply manifold thereby controlling the separation of the source water between the supply manifold of 01 and the NF supply manifold and therefore the separation of the water from the NF supply manifold. origin between the membrane units of 01 and the membrane units of NF. If necessary, a pressure relief valve upstream of the NF feed manifold may also be provided. Alternatively, a control valve of the type described above may be provided upstream of the NF feed manifold.

In the second preferred aspect of the present invention, the source water may be at a pressure below the desired inlet pressure for the membrane units of 01. Therefore it is necessary to reinforce the supply water pressure of 01 using a reinforcing pump. Preferably, the booster pump is coupled to a hydraulic recovery system that recovers energy from the combined retained material stream leaving the membrane units of 01. This hydraulic recovery system can be a hydraulic turbine. Therefore, a shaft of the turbine can drive a tree of the booster pump. These trees can be connected through a gear system. However, one skilled in the art will understand that additional power must be supplied to the booster pump if the RO feed water has to reach the desired inlet pressure for the RO membrane units.

Normally, the source water is pressurized to a value in the range of 350 to 1100 psi absolute before splitting to provide the OI feed water and the NF feed water. It is preferred to pressurize the source water to a value above the inlet pressure for the NF membrane units before dividing the source water to give the OI and NF feed waters. Therefore, it is preferred that the source water pressure is in the range of 600 to 1100 psi absolute, preferably 700 to 900 psi absolute.

In this second preferred aspect of the present invention, each membrane train is provided with a first feed manifold for the RO feed water (the pressure of which has been boosted to a pressure in the range of 900 to 1250 psi absolute), second feed manifold for NF feedwater (whose pressure has normally been decreased to a pressure in the range of 350 to 450 psi absolute), a collector of retained material for a combined retentate stream and a permeate collector for a stream of combined permeate material. When a row only contains membrane units of 01, a common supply conduit is provided which leads from the supply manifold of 01 to the individual membrane units of 01 of each row. In a similar way, a common retained material flow conduit and a common permeate material flow conduit lead from the individual membrane units of 01 of each row to the collectors of retained material and permeate, respectively. When a row contains NF membrane units, a common supply conduit is provided for the NF membrane units leading from the NF feed manifold. Similarly, the NF membrane units of the row are provided with common retained material flow conduits and common permeate material leading to the collectors of retained material and permeate, respectively. When a row contains both NF membrane units and diaphragm units of 01, a specialized common supply conduit, a specialized common retained material conduit and a common commonized permeate material conduit for the units are provided. Membrane of 01. Similarly, a specialized common NF feed line, a specialized common NF retentate conduit and a specialized common NF permeate conduit are provided for the NF membrane units.

Similar to the first preferred aspect of the present invention, a flow controller is normally provided to control the separation of source water between the supply manifold of 01 and the NF supply manifold. Normally, a pressure relief valve is provided upstream of the NF supply manifold so that the pressure can be lowered to the desired inlet pressure for the NF membrane units. However, it can also be conceived that a pressure lowering valve can be provided in the or in each common NF feed conduit. Alternatively, as described above, a control valve may be provided upstream of the NF feed manifold thereby controlling both the source water separation and the NF feed water pressure.

Providing NF membrane units in each train of the desalination plant allows the plant to continue to operate and produce water of salinity, the concentration of sulfate anions and the concentration of multivalent cations desired in the event that it becomes necessary to turn off one or more of the trains for maintenance or cleaning.

Normally, the membrane (s) contained in each membrane unit in a row are provided with waterproof pressure fittings for connection to (i) the common supply duct, (ii) the flow duct common permeate material; and (iii) the common retained material feed line.

Suitably, a back pressure valve is provided in or in each permeate material flow conduit of common NF upstream of the mixing point for the permeate material of NF and the permeate material of .01. Alternatively, when there is more than one conduit of common NF permeate material, these conduits can lead to a permeate conduit of combined NF and the backpressure valve can be provided in this permeate conduit of combined NF. The back pressure valve ensures that the pressure of the NF permeate material is sufficiently greater than the pressure of the RO permeate material to allow the permeate material of NF to be injected into the permeate material collector. The resulting mixed permeate stream is the injection water stream which then enters an injection water flow conduit. Suitably, the back pressure valve opens when the NF permeate material pressure exceeds a preset pressure and allows a sufficient flow of NF permeate material through the valve to maintain the NF permeate material pressure greater than the pressure pre-established Normally, the preset pressure of the back pressure valve is at least 5 psi higher than the pressure of the permeate material of 01. Generally, the pressure of the permeate material of 01 will be in the range of 10 to 75 psi absolute, preferably 20 to 55 psi absolute.

Preferably, the source water may have been subjected to at least one of: filtration to remove particulate matter, chlorine removal, dosage of a biocide, deaeration and dosing of an scale inhibitor. These treatments can be carried out in the first and / or NF feed waters, but in order to reduce the space and the weight of the plant, it is preferred to carry out these treatments in the source water supply before dividing the water of origin to form the OI feed water and the NF feed water.

As an alternative to deaerate the source water upstream of the desalination plant, it is conceived that a deaerator can be provided downstream of the desalination plant in order to control corrosion in the injection ducts, injection pumps and wells of injection. An advantage of providing a downstream deaerator is that the volume of water that is subjected to deaeration is substantially less than if the deaerator were disposed upstream of the desalination plant. However, having a de-aerator upstream of the desalination plant reduces the risk of corrosion within the desalination plant and therefore allows the use of cheaper steels. Therefore, it may be advantageous to provide a deaerator upstream of the desalination plant.

In a further embodiment of the present invention there is provided a desalination plant comprising a plurality of trains, each comprising a plurality of membrane units of 01 and a plurality of membrane units of NF, in which the ratio of units of membrane of 01 with respect to NF membrane units in each membrane train is in the range of 2: 1 to 40: 1, preferably 4: 1 to 27: 1, in particular 10: 1 to 25: 1, and in which each membrane train is provided with: (a) a supply conduit for a source water, the supply conduit being divided to provide a supply conduit (or manifold) for the RO membrane units and a supply conduit (or manifold) for the membrane units of NF, (b) a conduit of permeate material (or collector) for the membrane units of 01 and a conduit of permeate material (or collector) for the membrane units of NF, the conduits of permeate material being combined to provide a water conduit of injection; (c) a retentate material conduit (or collector) for the membrane units of 01 and a retentate material conduit (or manifold) for the NF membrane units; Y (d) a flow controller and a pressure drop valve in the NF feed line.

As discussed above, it is conceived that the flow controller and a pressure decrease valve can be combined in the form of a control valve.

Preferably, the membrane units are arranged in rows placed one above the other. Preferably, the NF membrane units are arranged together in one or more rows. Normally, each membrane train comprises between 3 and 15 rows, each row comprising between 4 and 16 membrane units.

Preferably, a booster pump is provided in the supply conduit of 01 and a hydraulic recovery unit in the RO material conduit, the hydraulic recovery unit being coupled to the booster pump. Normally, the hydraulic recovery unit is a hydraulic turbine of the type described above. Alternatively, the hydraulic recovery unit may be a turbocharger.

The capacity of the desalination plant must be sufficient to meet the requirements of low salinity injection water for the oil field. Normally, each train of the desalination plant can produce between 20,000 and 200,000 barrels of water per day, for example, between 40,000 and 60,000 barrels of water per day of the desired low salinity and the desired low concentration of sulfate anions.

Preferably, a back pressure valve is provided in the NF permeate material conduit (s) to allow accurate metering of the NF permeated material into the permeated RO material, thereby resulting in the production of a water of injection having the desired characteristics, for example desired controlled salinity, desired concentration of sulfate anions and desired multivalent cation concentration.

BEREVE DESCRIPTION OF THE FIGURES The present invention will be described with reference to the following examples and figures, in which: Figure 1 is a schematic diagram of the process and the desalination plant of the present invention, Figure 2 is a schematic diagram of a modification of the process and the desalination plant of the present invention, and Figure 3 is a schematic diagram of a train of membrane units for use in the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION In Figure 1, a source water supply 1 is provided for a desalination plant comprising a plurality of OI membrane units (shown schematically in 2) and a plurality of NF membrane units (shown schematically in 3) to a pump 4 which increases the pressure of source water 1 to a desired value in the range of 900 to 1250 pounds per square inch absolute (psi absolute). Preferably, the source water has been treated upstream of pump 4. Therefore, the source water may have been chlorinated, purified and passed through a filtration system to remove the particulate matter to below a desired level, normally up to a level commensurate with a Silt density index (SDI 15 minutes) of less than 5 and preferably less than 3. The reduction of SDI can be achieved using a variety of well-known methods including microfiltration, ultrafiltration, filtering media systems and filtration of cartridge. A chlorine scavenger downstream of the filtration system can be dosed to the filtrate to remove any residual free chlorine that would otherwise damage the membranes of the membrane units that are disposed downstream of the pump 4. The source water can also be discharged. passing through a deaerator to remove oxygen, thereby controlling corrosion in the desalination plant and downstream of the desalination plant, for example, in the injection ducts, injection pumps and injection wells . If desired, a biocide upstream of the pump 4 can also be dosed to the source water in order to control the biological activity that might otherwise occur in the system. An incrustation inhibitor can also be dosed into the source water upstream of the pump 4 in order to minimize fouling on the downstream membrane surfaces.

The source water 1 is divided downstream of the pump 4 to provide a feed water 5 of 01 for the plurality of OI membrane units 2 and an NF feed water 6 for the plurality of NF membrane units 3 . Preferably, these membrane units are disposed in one or more membrane streams which are described in more detail below with reference to Figure 3. The separation of the RO feed water 5 and the NF feed water 6 is controlled by the flow controller 7 The pressure of the NF feed water 6 is then reduced to a value in the 350 to 450 psi absolute range through a pressure drop valve 8 before being fed to the NF membrane units 3. The retained material 9 of OI extracted from the OI membrane units 2 and the retained material 10 of NF extracted from the NF membrane units 3 is rejected while the permeated material 11 of OI extracted from the OI membrane units 2. and the NF permeated material 12 extracted from the NF membrane units 3 combine to provide a controlled salinity injection water 13 and controlled sulfate anion concentration.

Figure 2 is a modification of the process and the desalination plant of Figure 1 in which the pump 4 increases the pressure of the source water to a value of 700 psi absolute before dividing the source water into a feed water 5 of OI and a water 6 of NF feed.

The pressure of the feed water 5 of 01 is then reinforced for the plurality of membrane units 2 from 01 to the desired operating pressure of the membrane units 2 of 01 (1100 psig) using a booster pump 14 thereby generating a Water 16 of supply 01 pressurized. A hydraulic recovery turbine 1 is coupled to the booster pump 15 and recovers energy from the retained material 9 which is withdrawn from the membrane units 9 of 01 thereby generating a retained reduced pressure material 17 which is rejected from the desalination plant. . The pressure of the NF feed water 6 is reduced to a value in the 350 to 450 psi absolute range through a pressure decrease valve 8 as described with respect to Figure 1.

Figure 3 illustrates a cross-section through a membrane train 20 for use in the process and desalination plant of the present invention. The membrane train 20 comprises seven rows 21, each row comprising eight membrane units 22 arranged in substantially horizontal planes one above the other. However, it is conceived that a membrane train may comprise more than or less than seven rows and each row may comprise more than or less than eight membrane units. Each of the membrane units comprises a housing having a substantially cylindrical shape having a length in the range of 35 to 345 inches (from 0.89 to 8.76 meters), and an internal diameter in the range of 2.5. to 75 inches (from 6. 35 cm to 1.91 meters). The housing contains at least one spirally wound membrane (not shown), preferably two to four spirally wound membranes, preferably three or four spirally wound membranes. Each of the spirally wound membranes is rolled into a cylinder and has a length in the range of 30 to 60 inches (from 0.762 to 1.52 meters) and an outer diameter in the range of 2.5 to 18 inches. (from 6. 36 to 45.7 cm). A typical membrane has a length of approximately 40 inches (1.02 meters) and a diameter of approximately 8 inches (20.3 cm). When a housing contains more than one membrane, the membranes are usually arranged end-to-end, in which case the housing generally has an internal diameter of up to 18 inches (45.7 cm) and a length of up to 345 inches (8.76). meters).

The membrane train 20 has a feed collector 23a for the water of origin of OI and a feed collector 23b for the feed water of NF. The OR supply manifold 23a is normally disposed substantially vertically, at a midpoint of the train so that half of the RO membrane units in each row are disposed on each side thereof. For example, when each row of membranes has eight membrane units of 01, four membrane units of 01 can be provided on each side of the supply manifold 23a of 01. Most of the membrane units in the train are reverse osmosis units ( 01) and the remaining ones are nanofiltration membrane units (NF), the ratio of units of 01 depending on NF units of the desired mixing ratio of permeate material of 01 and permeate material of NF, which in turn depends of the volume recovery in% of the permeate material from the membrane units of 01 and NF. Figure 3 shows four NF membrane units 24 arranged in the lower row to the left of the NF feed collector 23b. However, it is also conceived that the NF feed collector 23b can be arranged at the midpoint of the train.

A plurality of common feed conduits leads from the feed collectors to the rows of the membrane train. Therefore, the bottom row is provided with a first common supply conduit 25 leading from the supply manifold 23a of 01 to the four membrane units of 01 arranged to the left of the supply manifold 23a of 01 and a second conduit 26 common feed that leads from the NF feed collector 23b to the four NF membrane units 24 disposed to the left of the NF feed collector 23b. Therefore, the water flowing through the second common supply conduit 26 is the NF water feed. A flow control valve and a pressure decrease valve (not shown) are provided in the second common supply conduit 26 for reducing the pressure of the NF water supply to the operating pressure of the membrane units 24 of NF The pressure decrease valve is controlled through a pressure controller (not shown) so that the pressure of the NF feed water downstream of the valve is in the range of 350 to 450 psi absolute. The NF membrane units 24 are single phase single pass units, the retained material from the NF membrane units being rejected by a common retentate reject conduit (not shown) leading to a NF retained material collector. (not shown). The permeate material from each NF membrane unit is fed to a common NF permeate material conduit (not shown) leading to a collector of NF permeate material (not shown).

The remaining rows of the train (the six upper rows) are each provided with a common supply conduit 27 leading from the supply manifold 23a of 01 to each of the membrane units of 01 of the row. Therefore, the water flowing through the first common supply conduit 25 of the lower row and the common supply conduits 27 of the six upper rows is the supply water of 01 for the membrane units of 01. As with the the membrane units of NF, the membrane units of 01 shown in figure 3 are single-step single-phase units. However, as discussed above, the membrane units of the train 01 may also be single-pass units two phases. The person skilled in the art will understand how to modify the train of Figure 3 so that the membrane units of 01 are operated in a two-step single-pass mode. The retained material from the membrane units of 01 of each row is rejected by feeding through a retentate conduit of common retained material (not shown) to the retentate collector of 01 (not shown). The retained NF material and the retained material of 01 are optionally combined and either discharged into the environment, for example, to the sea, or injected into an injection well either in a hydrocarbon-bearing formation or in an aquifer. The permeate material from the membrane units of 01 of each row is fed through a common permeate material conduit (not shown) to a permeate material collector (not shown) where it is combined with the NF permeate material. . The NF permeate material conduit is provided with a back pressure valve to ensure that the pressure of the NF permeate material is sufficiently above that of the RO permeate material so that the NF permeate material can be dosed into the material manifold permeate and mix with the permeated RO material thereby forming the injection water stream.

Example 1 A low salinity injection water stream can be prepared from a source water that has a TDS content of 35,800 ppm, an anion concentration sulphate of 2,750 ppm and a concentration of multivalent cations (sum of the concentrations of calcium and magnesium cations) of 1830 ppm feeding the source water at a rate of 320 thousand barrels of water per day (mbwd) to a desalination plant comprising a plurality of RO membrane units and a plurality of NF membrane units. The source water feed was divided to provide an OI feed water for the OI membrane units (310 mbwd) and an NF feed water for the NF membrane units (10 mbwd). The OI membrane units were operated at a pressure of 1000 psi absolute and at a 50% recovery in volume to provide 155 mbwd of a stream of permeated RO material having a TDS content of 177 ppm, a sulfate anion concentration of 1.5 ppm and a multivalent cation concentration of 2.5 ppm. The NF feed water pressure was reduced for the NF membrane units through a pressure decrease valve to a pressure of 400 psi absolute (the operating pressure of the NF membrane units). The NF membrane units were operated at a 50% by volume recovery to provide 5 mbwd of an NF permeate material stream having a total dissolved solids content of 26,500 ppm, a sulfate concentration of 25 ppm and a concentration of multivalent cations of 132 ppm. The NF permeate material stream and the RO permeate material stream were combined to give 160 mbwd of an injection water stream having a TDS content of 1000 ppm, a sulfate anion concentration of 2.2 ppm and a multivalent cations concentration of 6.5 ppm (using a combination ratio of permeate material of OI with respect to material permeate of NF of 31.0: 1).

Example 2 A low salinity injection water stream can be prepared from a source water having a TDS content of 35,800 ppm, a sulfate anion concentration of 2,750 ppm and a concentration of multivalent cations (sum of cation concentrations). calcium and magnesium) of 1830 ppm by feeding the source water at a rate of 320 thousand barrels of water per day (mbwd) to a desalination plant comprising a plurality of RO membrane units and a plurality of NF membrane units . The source water feed was divided to provide an OI feed water for the OI membrane units (261.4 mbwd) and an NF feed water for the NF membrane units (58.6 mbwd). The OI membrane units were operated at a pressure of 1000 psi absolute and a 50% recovery in volume to provide 130.7 mbwd of a stream of permeated RO material having a TDS content of 177 ppm, a concentration of sulfate anions of 1.5 ppm and a concentration of multivalent cations of 2.5 ppm. The NF feed water pressure was reduced for the NF membrane units through a pressure decrease valve to a pressure of 400 psi absolute (the operating pressure of the NF membrane units). The NF membrane units were operated at a 50% by volume recovery to provide 29.3 mbwd of an NF permeate material stream having a total dissolved solids content of 26,500 ppm, a sulfate concentration of 25 ppm and a concentration of multivalent cations of 132 ppm. The permeate stream of NF and the permeate stream of RO were combined to give 160 mbwd of an injection water stream having a TDS content of 5000 ppm, a sulfate anion concentration of 5.8 ppm and a multivalent cation concentration of 26.2 ppm (using a combination ratio of permeate material of OI with respect to material permeate of NF of 4.5: 1).

Claims

NOVELTY OF THE INVENTION Having described the present invention, it is considered a novelty and, therefore, what is contained in the following is claimed as property. CLAIMS 1. Process for producing a controlled salinity injection water stream and controlled sulfate anion content which is suitable for injection into an oil formation of an oil field, the method comprising the steps of: (a) feeding a source water having a total dissolved solids content in the range of 20,000 to 45,000 ppm and a concentration of sulfate anions in the range of 1,000 to 4,000 ppm, preferably from 1,500 ppm to 4,000 ppm to a plant of desalination comprising a plurality of reverse osmosis membrane (RO) units and a plurality of nanofiltration membrane (NF) units, wherein the source water is pressurized to a pressure in the range of 350 to 1250 psi absolute, and dividing the source water to provide a feed water for the RO membrane units (hereinafter "RO feed water") and a feed water for the NF membrane units (as successively in the present document "NF feed water"); (b) if necessary, increase the pressure of the RO feed water to a value in the range of 900 to 1250 psi absolute before introducing the RO feed water into the RO membrane units and removing a permeate material from the RO. OI and an OI retained material of the OI membrane units, wherein the OI membrane units are operated either in a single-pass, single-phase or single-pass mode, two phases and wherein the permeate RO material recovery is in the range of 35 to 75% by volume, preferably 35 to 60% by volume based on the volume of the OI feed water that is fed to the OI membrane units , so that the permeated RO material has a total dissolved solids content of less than 250 ppm, and a sulfate anion concentration of less than 3 ppm; (c) if necessary, reduce the pressure of the NF feed water to a value in the 350 to 450 psi absolute range before introducing the NF feedwater into the NF membrane units and removing a permeate material from the NF membrane. NF and an NF retained material of the NF membrane units, wherein the NF membrane units are operated in a single-pass, single-phase mode and in which the NF membrane units are operated with a recovery of permeate material of NF in the range of 35 to 60% by volume based on. the volume of the NF feedwater that is fed to the NF membrane units, so that the permeate material of NF has a total dissolved solids content in the range of 15,000 to 40,000 ppm, preferably 15,000 to 35,000 ppm, and a sulfate anion concentration less than 40 ppm, preferably less than 30 ppm; Y (d) mixing at least a portion of the RO permeated material and at least a portion of the NF permeated material in a ratio in the range of from 2: 1 to 40: 1, preferably from 4: 1 to 27: 1, in in particular from 10: 1 to 25: 1 to provide an injection water having a total dissolved solids content in the range of 500 to 5,000 ppm, preferably 1,000 to 5,000 ppm, and a sulfate anion concentration of less than 7.5 ppm, preferably less than 5 ppm, more preferably less than 3 ppm. 2. Process according to claim 1, wherein the source water has a concentration of multivalent cations in the range of 700 to 3,000 ppm, preferably from 1,500 to 2,500 ppm, the permeated RO has a multivalent cation content of up to 10 ppm , the NF permeate material has a multivalent cation content of up to 200 ppm, preferably up to 150 ppm; and the injection water has a multivalent cation content of up to 50 ppm. 3. The method according to claim 1 or 2, wherein energy is recovered from the retentate of the OI using a hydraulic recovery unit and wherein the pressure of the OI feed water is increased in step (b) using a booster pump that it is coupled to the hydraulic recovery unit. 4. Process according to any one of the preceding claims, wherein the pressure of the NF feed water in step (c) is reduced to a pressure in the range of 350 to 450 psi absolute, preferably 380 to 420 psi absolute, per means of a pressure reduction valve. 5. Process according to any one of the preceding claims, wherein the source water is selected from seawater, estuarine water, a produced water, an aquifer water, and a waste water. 5. A process according to any one of the preceding claims, wherein the total dissolved solids content of the permeated OI material is in the range of 50 to 225 ppm and the sulfate anion content of the permeated OI material is at least 0.5. . 7. Process according to any one of the preceding claims, wherein the total dissolved solids content of the NF permeate material is not more than 15,000 ppm lower, preferably not more than 10,000 ppm lower than the total dissolved solids content of the source water and in that the permeate material of NF has a sulfate anion concentration of at least 10 ppm. 8. Process according to any one of claims 2 to 7, wherein the concentration of multivalent cations in the permeated material of OI is in the range of 1 to 10 ppm and the concentration of multivalent cations in the permeate material of NF is in the range from 50 to 200 ppm. 9. Process according to any one of the preceding claims, wherein one or more of the flow velocity of the source water, the mixing ratio of the permeate material of RO and the permeate material of NF, and the flow rate of the injection water it is determined according to a measured variable that is selected from one or more of the conductivity of the injection water, the total concentration of divalent anions in the injection water or in the permeate material of NF, and the concentration of sulfate anions in the water of injection or in the permeated material of NF. 10. Process according to any one of the preceding claims, wherein the pressure of the permeate material of NF is maintained at least 5 psi higher than the pressure of the permeate material of 01 by means of a back pressure valve which is provided upstream of the mixing point for the permeate material of NF and the permeate material of OI and in which the permeate material of NF is injected into the permeate material of OI at the mixing point to form the injection water. 11. Desalination plant comprising a plurality of membrane streams each comprising a plurality of RO membrane units and a plurality of NF membrane units in which the ratio of RO membrane units to NF membrane units in each membrane train is in the range from 2: 1 to 40: 1, preferably from 4: 1 to 27: 1, in particular from 10: 1 to 25: 1, and in which each membrane train is provided with : a) a supply conduit for a source water that is divided to provide a supply conduit for the RO membrane units and a supply conduit for the NF membrane units, b) a permeate material conduit for the membrane units of 01 and a permeate material conduit for the NF membrane units which combine to provide an injection water conduit; c) a retentate conduit for the membrane units of 01 and a retentate conduit for the NF membrane units; Y d) a flow controller and a pressure drop valve in the NF feed line. 12. Desalination plant according to claim 11, wherein a booster pump is provided in the supply conduit of 01 and a hydraulic recovery unit in the retained material conduit of 01 and in which the hydraulic recovery unit is coupled to the reinforcing pump. 13. Desalination plant according to claim 12, in which the hydraulic recovery unit is a hydraulic turbine having a shaft that is coupled to a drive shaft of the booster pump. 14. Desalination plant according to any one of claims 11 to 13, wherein the desalination plant comprises from 2 to 6, preferably from 2 to 4, membrane trains, each membrane train comprising between 3 and 15 rows, preferably between 6 and 12 rows and comprising each row between 4 and 16 membrane units 15. Desalination plant according to any one of claims 11 to 14, wherein the NF membrane units and the membrane units of 01 of each train are single phase units. 16. Desalination plant according to any one of claims 11 to 15, wherein a back pressure valve is provided in the permeate material conduit of NF. SUMMARY A process for producing a controlled salinity injection water stream and controlled sulfate anion content that is suitable for injection into an oil formation of an oil field, the method comprising the steps of: (a) feeding a water of origin that has a total dissolved solids content in the range of 20,000 to 45,000 ppm and a concentration of sulfate anions in the range of 1,000 to 4,000 ppm, preferably from 1,500 ppm to 4,000 ppm, to a desalination plant comprising a plurality of membrane units of reverse osmosis (LE) and a plurality of nanofiltration membrane (NF) units, in which the source water is pressurized to a pressure in the range of 350 to 1250 psi absolute, and divide the source water to provide a feed water for the membrane units of 01 (hereinafter referred to as "RO feed water") and feed water ation for the NF membrane units (hereinafter "NF feed water"); (b) if necessary, increase the supply water pressure from 01 to a value in the range of 900 to 1250 psi absolute before introducing the RO feed water into the RO membrane units and removing a permeate material from the RO membrane. OI and a retained material of 01 of the membrane units of 01, in which the membrane units of 01 are operated either in a single-pass mode, single phase or in a single-pass mode, two phases and wherein the permeate material recovery of 01 is in the range of 35 to 75% by volume, preferably from 35 to 60% by volume based on the volume of the feed water of 01 which is fed to the OI membrane units , so that the permeate material of 01 has a total dissolved solids content of less than 250 ppm, and a sulfate anion concentration of less than 3 ppm; (c) if necessary, reduce the pressure of the NF feed water to a value in the 350 to 450 psi absolute range before introducing the NF feedwater into the NF membrane units and removing a permeate material from the NF membrane. NF and an NF retained material of the NF membrane units, wherein the NF membrane units are operated in a single-pass, single-phase mode and wherein the NF membrane units are operated with a recovery of permeate material of NF in the range of 35 to 60% by volume based on the volume of the NF feedwater that is fed to the NF membrane units, so that the permeate material of NF has a content of total dissolved solids in the range of 15,000 to 40,000 ppm, preferably 15,000 to 35,000 ppm, and a sulfate anion concentration of less than 40 ppm, preferably less than 30 ppm; * and (d) mixing at least a portion of the RO permeate material and at least a portion of the NF permeate material in a ratio in the range of 2: 1 to 40: 1, preferably 4: 1 to 27: 1. , in particular from 10: 1 to 25: 1 to provide an injection water having a total dissolved solids content in the range of 500 to 5,000 ppm, and a sulfate anion concentration of less than 7.5 ppm, preferably less than (5) ppm, more preferably less than 3 ppm.